Does tree growth sensitivity to warming trends vary according to treeline form?

Abstract

AbstractAimWhereas many treelines are advancing coincident with climate warming, many other treelines are stationary. Differential sensitivity to warming trends may be partially accounted for by differences in the climatic limits associated with distinct treeline forms such as diffuse, abrupt and krummholz treelines. We tested the hypothesis that only diffuse treelines are strictly growth‐limited by low temperatures and thus should benefit from warming more than abrupt or krummholz treelines.LocationColorado Front Range, USAMethodsThe growth‐climate responses of trees growing at different treeline forms were examined. We built tree‐ring chronologies from seven sites covering diffuse, abrupt and krummholz treelines for two conifer species – Abies lasiocarpa and Picea engelmannii. These chronologies were correlated with climatic variables and compared in terms of growth trends.ResultsThe variance in tree‐ring width chronologies was primarily attributed to tree species and secondarily to treeline form. Tree growth at krummholz sites was limited by the length of the growing season, and ring widths of trees at abrupt treelines also showed a strong drought signal. The growth‐climate response of diffuse treelines varied by sites and trees, showing mostly a mixed climatic signal. In general, trees limited by the length of the growing season or by the growing season temperatures were characterized by the increasing growth rates during the last three decades, whereas trees limited by other factors displayed ambiguous growth trends.Main conclusionsRecent growth increase was most pronounced for Picea engelmannii at the krummholz treeline form. As a refinement of the treeline form framework, we suggest that temperature‐limited tree growth at krummholz treelines may be important, whereas the main common feature of diffuse treelines is their population dynamics and not temperature‐limited radial growth. Our results highlight the importance of considering differences in species and treeline form in projecting future treeline advances under a warming climate.